Reading and writing actuators which are moveable in focus and tracking positions include an objective lens. These actuators are often used in optical disc recording devices to control the objective lens position relative to rotating disc. This is necessary because of unavoidable warp of the disc, spindle errors, and other mechanical and optical imperfections. Both focus (controlled by the distance between the lens and the disc surface) and tracking (the correct radial position of the lens so as to read or write a single track at a time) must be maintained. The motions of the actuator are controlled by a servo system.
As shown in FIG. 1, typically, an actuator 9 receives a read/write light beam from a laser 10 directed by a reflective surface 12 to an actuator assembly 14 which includes an objective lens 14a. This limits the compactness of the actuator. The actuator assembly 14 is mounted on flexures 18 fixed to a base 20. The actuator assembly 14 typically has two magnetic assemblies (not shown) that supply the magnetic fields used by actuator motors. The actuator assembly 14 includes at least one position sensor (not shown) which has its own dedicated light source, commonly an LED. This arrangement adds cost, complexity, and reduces reliability.